The present invention relates to polymer electrolyte fuel cells used for portable power sources, electric vehicle power sources, domestic cogeneration systems, etc.
A fuel cell using a polymer electrolyte generates electric power and heat simultaneously by electrochemical reaction of a fuel gas containing hydrogen and a fuel gas containing oxygen such as air. This fuel cell is basically constructed by a pair of electrodes, namely, an anode and a cathode, formed on both surfaces of a polymer electrolyte membrane that selectively transports hydrogen ions. The above-mentioned electrode comprises a catalyst layer composed mainly of a carbon powder carrying a platinum group metal catalyst, and a diffusion layer which has both gas permeability and electronic conductivity and is formed on the outside surface of this catalyst layer.
Moreover, in order to prevent leakage of the fuel gas and oxidant gas supplied to the electrodes and prevent mixing of two kinds of gases, a gas sealing material and gaskets are arranged on the periphery of the electrodes with the polymer electrolyte membrane therebetween. These sealing material and gaskets are assembled into a single part together with the electrodes and polymer electrolyte membrane in advance. This part is called xe2x80x9cMEAxe2x80x9d (electrolyte membrane and electrode assembly). Disposed outside of the MEA are conductive separator plates for mechanically securing the MEA and for electrically connecting adjacent MEAs in series. A portion of the separator plate, which is in contact with the MEA, is provided with a gas passage for supplying a reacting gas to the electrode surface and for removing a generated gas and excess gas. Although the gas passage can be provided separately from the separator plate, it is usual to form a groove on a surface of each separator plate to serve as the gas passage.
In order to supply the fuel gas and oxidant gas to these grooves, it is necessary to branch pipes that supply the fuel gas and the oxidant gas, respectively, according to the number of separator plates to be used and to use piping jigs for connecting an end of the branch directly to the separator plate. This jig is called xe2x80x9cmanifoldxe2x80x9d and a type of manifold that directly connects the supply pipes of the fuel gas and oxidant gas to the grooves as mentioned above is called xe2x80x9cexternal manifoldxe2x80x9d. There is a type of manifold, called xe2x80x9cinternal manifoldxe2x80x9d, with a more simple structure. The internal manifold is configured such that through holes are formed in the separator plates having gas passages and the inlet and outlet of the gas passages are extended to the holes so as to supply the fuel gas and oxidant gas directly from the holes.
Since the fuel cell generates heat during operation, it is necessary to cool the cell with cooling water or the like in order to keep the cell in good temperature conditions. In general, a cooling section for feeding the cooling water is provided for every one to three cells. There are a type in which the cooling section is inserted between the separator plates and a type in which a cooling water passage is provided in the rear surface of the separator plate so as to serve as the cooling section, and the latter type is often used. The structure of a common cell stack is such that these MEAs, separators and cooling sections are placed one upon another to form a stack of 10 to 200 cells, and this stack is sandwiched by end plates, with a current collector plate and an insulating plate between the stack and each end plate, and secured with clamping bolts from both sides.
In such a polymer electrolyte fuel cell, the separator plates need to have a high conductivity, high gas-tightness for the fuel gas and oxidant gas, and high corrosion resistance against a reaction of hydrogen/oxygen oxidation-reduction. For such reasons, a conventional separator plate is usually formed from carbon material such as glassy carbon and expanded graphite, and a gas passage is produced by cutting a surface of the separator plate, or by molding with a mold when the expanded graphite is used.
In a conventional method of cutting a carbon plate, it was difficult to reduce the cost of the material of the carbon plate and the cost of cutting the carbon plate. Besides, a method using expanded graphite also suffers from a high cost of material, and it has been considered that the high cost of material prevents a practical application of this method.
In resent years, attempts to use a metallic plate, such as stainless steel, in place of a conventionally used carbon material have been made.
However, in the above-mentioned method using a metallic plate, since the metallic plate is exposed to oxidizing atmosphere of the pH of around 2 to 3 at high temperatures, the corrosion and dissolution of the metallic plate will occur when used in a long time. The corrosion of the metallic plate increases the electrical resistance in the corroded portion and decreases the output of the cell. Moreover, when the metallic plate is dissolved, the dissolved metal ions diffuse in the polymer electrolyte and are trapped at the ion exchange site of the polymer electrolyte, resulting in a lowering of the ionic conductivity of the polymer electrolyte. For these causes, when a cell constructed by using a metallic plate as it is for a separator plate was operated for a long time, there was a problem of gradual lowering of the power generating efficiency.
An object of the present invention is to improve a separator plate for use in a fuel cell and provide a separator plate which is formed of a metal material that can be easily processed, prevented from corroding and dissolving to maintain chemical inactivity even when its surface to come in contact with a gas is exposed to acidic atmosphere and has good conductivity.
The present invention provides a polymer electrolyte fuel cell comprising: a hydrogen-ion-conductive polymer electrolyte membrane; a pair of electrodes sandwiching the hydrogen-ion-conductive polymer electrolyte membrane therebetween; a conductive separator plate having a gas passage for supplying a fuel to one of the electrodes; and a conductive separator plate having a gas passage for supplying an oxidant to the other electrode, wherein each of the conductive separator plates is formed of a metallic plate with a conductive coat comprising conductive particles and glass, formed on a surface having the gas passage.
The glass used here is preferably low-alkali glass. If high-alkali glass is used, alkali ions in the glass diffuse into the polymer electrolyte through water when the fuel cell generates power. Moreover, if the alkali ions are trapped at the ion exchange site of the polymer electrolyte, the ionic conductivity of the polymer electrolyte is lowered. Thus, if long-term stability is taken into consideration, it is preferred to select a glass composition containing almost no alkali components. Preferred low-alkali glass compositions are as follows.
As the conductive particles, it is possible to use metals such as Au, Pt. Rh and Pd; conductive inorganic oxides such as RuO2; conductive inorganic nitrides such as TiN, ZrN and TaN; and conductive inorganic carbides such as TiC, WC and ZrC. Among them, RuO2 is most preferred in terms of conductivity and costs.
The mixing ratio of the glass and conductive particles forming a coat on a surface of the metallic plate can vary depending on the types of conductive particles; if the adhesion strength and conductivity of the coat are taken into consideration, the mixing ratio is preferably within a range of 50 to 90 wt % of glass and 10 to 50 wt % of conductive particles, and more preferably within a range of 70 to 90 wt % of glass and 10 to 30 wt % of conductive particles. If the conductive particles are less than 10 wt %, a sufficient conductivity cannot be obtained; if the conductive particles exceed 50 wt %, the coat has poor adhesion and strength.
Next, as a method for coating the metallic plate with a mixture of conductive particles and glass frit, the mixture is prepared in the form of a sufficiently dispersed slurry, ink, paste or the like by using an organic binder or the like. As the organic binder, it is preferred to use a cellulose-based resin or acrylic-based resin so as to prevent remaining of decomposed residues after a heat treatment. Examples of the application method include a screen printing method, a doctor blade method, a drawing method, an offset printing method, a spraying method, and a dipping method.
An appropriate thickness of the conductive coat formed as described above is within a range of from 3 to 20 xcexcm. If the conductive coat is thinner than 3 xcexcm, it does not ensure a sufficient corrosion resistance of the base metal, and, if the conductive coat is thicker than 20 xcexcm, the resistance is increased.